Rail-based modular disc nucleus prosthesis

ABSTRACT

A method and apparatus for repairing a damaged intervertebral disc nucleus in a minimally invasive manner utilizes a modular disc prosthesis preferably comprised of at least three modular segments and at least two rails that operably connect adjacent modular segments. In one embodiment, each modular segment includes a harder inner portion and a softer outer portion. Preferably, the rails operate to slidably connect and interlock adjacent modular segments. A stem portion of the rails that extends outside the periphery of the body of the prosthesis is removable after implantation such that the modular segments form an implanted unitary device that closely mimics the geometry of the disc nucleus cavity.

RELATED APPLICATIONS

The present invention is a divisional application of U.S. patentapplication Ser. No. 11/372,477, filed Mar. 9, 2006, which claimspriority to U.S. Provisional Patent Application No. 60/685,332, filedMay 24, 2005, U.S. Provisional Patent Application No. 60/700,459, filedJul. 19, 2005, and U.S. Provisional Patent Application No. 60/660,107,filed Mar. 29, 2005, the disclosures of which are hereby incorporated byreference. The present invention is also related to the U.S. applicationSer. No. 11/372,357, filed Mar. 9, 2006, now U.S. Pat. No. 7,267,690which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to an implantable prosthesis forrepairing damaged intervertebral discs. More particularly, the presentinvention relates to a rail-based modular disc nucleus prosthesis ofpredetermined size and shape.

BACKGROUND OF THE INVENTION

The spinal motion segment consists of a unit of spinal anatomy boundedby two vertebral bodies, including the two vertebral bodies, theinterposed intervertebral disc, as well as the attached ligaments,muscles, and the facet joints. The disc consists of the end plates atthe top and bottom of the vertebral bones, the soft inner core, calledthe nucleus and the annulus fibrosus running circumferentially aroundthe nucleus. In normal discs, the nucleus cushions applied loads, thusprotecting the other elements of the spinal motion segment. A normaldisc responds to compression forces by bulging outward against thevertebral end plates and the annulus fibrosus. The annulus consists ofcollagen fibers and a smaller amount of elastic fibers, both of whichare effective in resisting tension forces. However, the annulus on itsown is not very effective in withstanding compression and shear forces.

As people age the intervertebral discs often degenerate naturally.Degeneration of the intervertebral discs may also occur in people as aresult of degenerative disc disease. Degenerative disc disease of thespine is one of the most common conditions causing pain and disabilityin our population. When a disc degenerates, the nucleus dehydrates. Whena nucleus dehydrates, its ability to act as a cushion is reduced.Because the dehydrated nucleus is no longer able to bear loads, theloads are transferred to the annulus and to the facet joints. Theannulus and facet joints are not capable of withstanding their increasedshare of the applied compression and torsional loads, and as such, theygradually deteriorate. As the annulus and facet joints deteriorate, manyother effects ensue, including the narrowing of the interspace, bonyspur formation, fragmentation of the annulus, fracture and deteriorationof the cartilaginous end plates, and deterioration of the cartilage ofthe facet joints. The annulus and facet joints lose their structuralstability and subtle but pathologic motions occur between the spinalbones.

As the annulus loses stability it tends to bulge outward and may developa tear allowing nucleus material to extrude. Breakdown products of thedisc, including macroscopic debris, microscopic particles, and noxiousbiochemical substances build up. The particles and debris may producesciatica and the noxious biochemical substances can irritate sensitivenerve endings in and around the disc and produce low back pain. Affectedindividuals experience muscle spasms, reduced flexibility of the lowback, and pain when ordinary movements of the trunk are attempted.

Degeneration of a disc is irreversible. In some cases, the body willeventually stiffen the joints of the motion segment, effectivelyre-stabilizing the discs. Even in the cases where re-stabilizationoccurs, the process can take many years and patients often continue toexperience disabling pain. Extended painful episodes of longer thanthree months often leads patients to seek a surgical solution for theirpain.

Several methods have been devised to attempt to stabilize the spinalmotion segment. Some of these methods include: heating the annularregion to destroy nerve endings and strengthen the annulus; applyingrigid or semi-rigid support members on the sides of the motion segmentor within the disc space; removing and replacing the entire disc with agenerally rigid plastic, articulating artificial device; removing andreplacing the nucleus; and spinal fusion involving permanently fusingthe vertebrae adjacent the affected disc.

Until recently, spinal fusion has generally been regarded as the mosteffective surgical treatment to alleviate back pain due to degenerationof a disc. While this treatment is often effective at relieving backpain, all discal motion is lost in the fused spinal motion segment. Theloss of motion in the affected spinal segment necessarily limits theoverall spinal mobility of the patient. Ultimately, the spinal fusionplaces greater stress on the discs adjacent the fused segment as thesesegments attempt to compensate for lack of motion in the fused segment,often leading to early degeneration of these adjacent spinal segments.

Current developments are focusing on treatments that can preserve someor all of the motion of the affected spinal segment. One of thesemethods to stabilize the spinal motion segment without the disadvantagesof spinal fusion is total disc replacement. Total disc replacement is ahighly invasive and technically demanding procedure which accesses thedisc from an anterior or frontal approach and includes dividing theanterior longitudinal ligament, removing the cartilaginous end platesbetween the vertebral bone and the disc, large portions of the outerannulus and the complete inner nucleus. Then an artificial total discreplacement is carefully placed in the evacuated disc space. Many of theartificial total disc replacements currently available consist of agenerally rigid plastic such as ultra high molecular weight polyethylene(“UHMWPE”) as the nucleus that is interposed between two metal platesthat are anchored or attached to the vertebral endplates. A summary ofthe history of early development and designs of artificial discs is setforth in Ray, “The Artificial Disc: Introduction, History andSocioeconomics,” Chpt. 21, Clinical Efficacy and Outcome in theDiagnosis of Low Back Pain, pgs. 205-225, Raven Press (1992). Examplesof these layered total disc replacement devices are shown, for example,in U.S. Pat. Nos. 4,911,718, 5,458,643, 5,545,229 and 6,533,818.

These types of artificial total discs have several disadvantages. First,because the artificial disc replacements are relatively large, theyrequire relatively large surgical exposures to accommodate theirinsertion. The larger the surgical exposure, the higher the chance ofinfection, hemorrhage or even morbidity. Also, in order to implant theprosthesis, a large portion of the annulus must be removed. Removing alarge portion of the annulus reduces the stability of the motionsegment, at least until healing occurs around the artificial disc.Further, because the devices are constructed from rigid materials, theycan cause serious damage if they were to displace from the disc spaceand contact local nerve or vascular tissues. Another disadvantage isthat rigid artificial disc replacements do not reproduce natural discmechanics.

An alternative to total disc replacement is nucleus replacement. Like anartificial disc prosthesis, these nucleus replacements are also inert,non-rigid, non-biological implants. The procedure for implanting anucleus replacement is less invasive than the procedure for a total discreplacement and generally includes the removal of only the nucleus andreplacement of the nucleus with a prosthesis that may be elasticallycompressible and provide cushioning that mimics a natural disc nucleus.Examples of implants used for nucleus replacement include: U.S. Pat.Nos. 4,772,287, 4,904,260, 5,192,326, 5,919,236 and 6,726,721.

Nucleus replacements are intended to more closely mimic natural discmechanics. To that end, some nucleus replacements utilize hydrogelsbecause of their water imbibing properties that enable thesereplacements to expand in situ to permit a more complete filling of theevacuated nucleus cavity. However, there is usually a trade-off in thatthe more expansion the hydrogel achieves, the less structural supportthe end product can provide. As a result, many hydrogel nucleus discreplacements have generally adopted the use of some form of a jacket orfabric to constrain the hydrogel material. For example, the implantdescribed in U.S. Pat. Nos. 4,772,287 and 4,904,260 consists of a blockof hydrogel encased in a plastic fabric casing. The implant described inU.S. Pat. No. 5,192,326 consists of hydrogel beads enclosed by a fabricshell. Without the jacket or other form of constraint, the hydrogel issusceptible to displacement because of the slippery nature of thehydrogel. Unfortunately, the jacket or fabric shell will be subject tolong term abrasive wear issues that could result in failure of thejacket or shell's ability to constrain the hydrogel and thus thehydrogel may be subject to displacement.

Another approach to nucleus replacement involves implantation of aballoon or other container into the nucleus, which is then filled with abiocompatible material that hardens in situ. Examples of this in situapproach to nucleus replacement include U.S. Pat. Nos. 6,443,988 and7,001,431. One of the problems with this approach is that the chemicalhardening process is exothermic and can generate significant amounts ofheat that may cause tissue damage. In addition, there is a possibilitythat the balloon may rupture during expansion, causing leakage ofmaterial into the disc cavity and surrounding tissues, which may causeundesirable complications.

Another technique for nucleus replacement involves implanting amultiplicity of individual support members, such as beads, one at a timein the evacuated disc nucleus cavity until the cavity is full. Examplesof this approach include U.S. Pat. Nos. 5,702,454 and 5,755,797. Becauseeach of the individual support members or beads is relatively small,there is a possibility that one or more of the individual supportmembers or beads may extrude out of the evacuated disc nucleus cavity.From a mechanical perspective, this technique is limited in the abilityto produce consistent and reproducible results because the location andinteraction of the multiplicity of beads or support members is notcontrolled and the beads or support members can shift during and afterimplantation.

Accordingly, there is a need for a nucleus prosthesis that may beinserted using a minimally invasive procedure and that mimics thecharacteristics of a natural disc.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for repairing adamaged intervertebral disc nucleus in a minimally invasive manner witha modular disc prosthesis. The modular disc prosthesis preferablycomprises at least three modular segments and at least two rails thatoperably connect adjacent modular segments. This configuration allowsthe prosthesis to be adapted for implantation through various surgicalapproaches, although the preferred method is the posterolateral(“posterior”) approach where the disc is accessed through the patient'sback. In one embodiment, each modular segment includes a harder innerportion and a softer outer portion. Preferably, the rails operate with asliding mechanism to connect and interlock adjacent modular segments. Astem portion of the rails that extends outside the periphery of the bodyof the prosthesis is removable after implantation such that the modularsegments form an implanted unitary device that closely mimics thegeometry of the disc nucleus cavity.

In one embodiment, a modular disc prosthesis that is adapted to beimplanted in an evacuated disc nucleus cavity includes at least threemodular segments each having a width. The first modular segment has afirst rail extending at least partially along one side of the width andbeyond a periphery of the first modular segment. The second modularsegment is slidably connected to the first rail on one side of the widthand has a second rail extending at least partially along another side ofthe width and beyond a periphery of the second modular segment. Thethird modular segment is slidably connected to the second rail on oneside of the width. The prosthesis has an expanded position in which themodular segments are extended along the first and second rails andpositioned in a generally end to end configuration spaced apart by therails prior to implantation. The prosthesis also has an implantedposition in which the modular segments are positioned in a generallyside by side configuration that defines a unitary body having agenerally continuous periphery that generally corresponds to theevacuated disc nucleus cavity with at least a portion of the railsextending beyond the periphery of the body.

Preferably, each modular segment comprises an inner portion and an outerportion. The inner portion includes structure that mates with one of therails. The outer portion substantially surrounds the inner portion,except for the side having one of the rails and the side havingstructure that mates with one of the rails. In one embodiment, the innerportion of each modular segment and the outer portion of each modularsegment are made of polymers of different durometers. Preferably, theinner portion of each modular segment has a compressive modulus fromabout 70-100 MPa and the outer portion of each modular segment has acompressive modulus from about 6-20 MPa. The use of a harder innerportion and softer outer portion as part of an integrated unitaryimplanted device permits the modular prosthesis of the present inventionto more closely mimic the stress response of a biological disc nucleuswhile simultaneously permitting effective operation of the slidablerelationship between adjacent modular segments.

In one embodiment, locking features are provided to ensure that themodular disc prosthesis is a unitary device both before and afterinsertion. To prevent the device from being separated prior toinsertion, locking features may be provided on the rigid rails toprevent modular segments from being slid back off of the rails. Thisensures that each modular segment is connected in its proper positionand in the proper order. In addition, locking features may be providedon the modular segments to lock them together upon insertion. Thisprevents individual segments from dislocating from the assembledprosthesis and migrating outside of the annulus.

Another aspect of the present invention comprises a method forimplanting a modular disc prosthesis. Because the modular discprosthesis may be implanted one segment at a time, a hole made in theannulus for implantation of the prosthesis may be a fraction of the sizeof the device in its final assembled form. The first modular segment isinserted into the disc nucleus space through the small hole in theannulus. The second modular segment is then slid up the first rigid railand into the disc nucleus space until the second modular segmentinterlocks with the first modular segment. The tail stem of the firstrigid rail is then severed from the device. Subsequent modular segmentsare slid up the adjoining rigid rail into the disc nucleus space andthen interlocked with the previously inserted modular segment in asimilar manner. Once all of the modular segments have been inserted andall of the tail stems severed, the modular disc prosthesis is fullyinserted into the patient's disc nucleus space.

Another aspect of the present invention provides an insertion tool thatmay be used to aid in the insertion, positioning, and rail removal ofthe modular prosthesis. The proximal end of the tool has a handle withan enclosed ratchet or roller mechanism attached to and in line with theinner lumen of an elongated tube at the distal end of the tool throughwhich a rail may be inserted. The elongated tube may have a slit orother openings along the length of the tube to aid in threading therails into the tube. Insertion tool may be provided with a cuttingmechanism for removing the rails from the modular segments once they arefully inserted.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a modular disc prosthesis accordingto the preferred embodiment of the present invention in its insertedconfiguration.

FIG. 2 is a top view of a modular disc prosthesis according to thepreferred embodiment of the present invention prior to insertion.

FIG. 3 is a perspective view of a modular disc prosthesis according toan alternate embodiment of the present invention prior to insertion.

FIG. 4 is a perspective view of a modular disc prosthesis according toan alternate embodiment of the present invention at a first stage ofinsertion.

FIG. 5 is a perspective a view of a modular disc prosthesis according toan alternate embodiment of the present invention at a second stage ofinsertion.

FIG. 6 is a perspective view of a modular disc prosthesis according toan alternate embodiment of the present invention at a final state ofinsertion.

FIG. 7 is a partial perspective view of a portion of a modular discprosthesis according to an embodiment of the present invention.

FIG. 8 is a view of an insertion tool for use in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there can be seen a cross-sectional view of amodular disc prosthesis 100 according to the preferred embodiment of thepresent invention as configured once inserted into the body. In thisembodiment, modular disc prosthesis 100 comprises first 102, second 104,third 106, fourth 108, and fifth 109 modular segments. Preferably, eachmodular segment 102, 104, 106, 108, 109 comprises a soft outer portion102 a, 104 a, 106 a, 108 a, 109 a and a hard inner portion 102 b, 104 b,106 b, 108 b, 109 b.

In a preferred embodiment, hard inner portions 104 b, 106 b and 108 bhave an I-beam cross-sectional shape that optimizes flexibility andstrength of the hard inner portions. Alternatively, hard inner portions104 b, 106 b, 108 b, can have a uniformly shaped cross-sectional area toreduce any differences in compressibility of the modular disc prosthesis100 across the surface area in order to minimize the potential forstress risers to be created in the interface between the outer surfaceof the modular disc prosthesis 100 and the inner surfaces of the discspace cavity. It will be recognized that various cross-sectional shapesof hard inner portions 102 b, 104 b, 106 b, 108 b and 109 b can beutilized in accordance with the present invention and that thecross-sectional shapes of the hard inner portions do not need to besymmetric.

Hard inner portion 102 b of first modular segment 102 includes firstsegment interlocking portion 116. Hard inner portion 104 b of secondmodular segment 104 includes second segment interlocking portion 118 anda first slot 128. Hard inner portion 106 b of third modular segment 106includes third segment interlocking portion 120 and a second slot 130.Hard inner portion 108 b of fourth modular segment 108 includes fourthsegment interlocking portion 121 and a third slot 132. Hard innerportion 109 b of fifth modular segment 109 includes a fourth slot 133.

In the preferred embodiment, as shown in FIG. 2, rails 110, 112, 114,115 have a noncircular cross-sectional shape, although it will beunderstood that other cross-sectional shapes could be utilized and thatthere is no requirement that all of the rails have similarcross-sectional shapes. It has been found that the noncircularcross-sectional shape as shown (corresponding mating C and sideways Tcross-sectional shapes) provides for better alignment of the modularsegments and supports larger insertion forces along the axis of therail.

It will be understood that in the preferred embodiment, the rails 110,112, 114, 115 of the present invention have a non-uniformcross-sectional aspect ratio in terms of the height and width of therail. Preferably, the rails have a relative rigidity along alongitudinal axis of the rail in a dimension of the height of the railthat is greater than a width of the rail, whereas in a dimensiontransverse to the width of the rail the relative rigidity of the railpermits a greater degree of flexibility such that succeeding modularsegments can be moved laterally with respect to one another in theexpanded position without deforming the rails. Preferably, the height ofthe rail is in the range of about 1 mm to 6 mm and the width of the railis in the range of about 0.5 mm to 4 mm. This differential rigidity inthe two dimensions transverse to the longitudinal axis of the rail isimportant in permitting effective and efficient sliding operation of theadjacent modular segments.

Referring to FIG. 2, there can be seen a portion of a modular discprosthesis 100 according to the preferred embodiment of the presentinvention prior to insertion into the evacuated disc nucleus cavity.Note that in FIG. 2, modular disc prosthesis 100 is depicted showingonly the hard inner portion 102 b, 104 b, 106 b, 108 b, 109 b of eachmodular segment 102, 104, 106, 108, 109 for convenience of illustration.However, in the preferred embodiment of the invention each modularsegment would also have a soft outer portion as described above andshown in FIG. 1.

In alternate embodiments, the modular disc prosthesis may comprisegreater or fewer numbers of modular segments and rails, as long as thereare at least three modular segments and two rails. For example, FIG. 3depicts a modular disc prosthesis 200 having four modular segments andthree rails.

Prior to insertion, modular disc prosthesis 100 further includes first110, second 112, third 114, and fourth 115 rails. First modular segment102 is rigidly attached to first rail 110 at first segment interlockingportion 116. Second modular segment 104 is slidably attached to firstrail 110 at first slot 128 and rigidly attached to second rail 112 atsecond segment interlocking portion 118. Third modular segment 106 isslidably attached to second rail 112 at second slot 130 and rigidlyattached to third rail 114 at third segment interlocking portion 120.Fourth modular segment 108 is slidably attached to third rail 114 atthird slot 132 and rigidly attached to fourth rail 115 at fourth segmentinterlocking portion 121. Fifth modular segment 109 is slidably attachedto fourth rail 115 at fourth slot 133.

As shown in FIG. 2 and FIG. 3, each rail 110, 112, 114 and 115 or 210,212 and 214 includes a stem portion that extends beyond a periphery ofthe body of the prosthesis 100, 200, respectively. Preferably these stemportions are long enough to permit access into the evacuated discnucleus space such that one modular segment can be positioned inside theevacuated disc nucleus cavity while the next modular segment on the railis still outside of the body. In an exemplary embodiment, the length ofthe stem portions ranges between 6 cm-20 cm.

As shown in the alternate embodiment of FIG. 3, each rail 210, 212, 214may further include a retaining portion 222, 224, 226 to keep the devicefrom being separated prior to insertion. The retaining portions 222, 224and 226 are configured to prevent the corresponding modular segments204, 206 and 206 from sliding off the rails. The retaining portions maybe molded into the rails or may be separate pieces or deformations ofthe rails added during the manufacture of the device.

The preferred embodiment is a unitary prosthesis that is packaged,sterile, and ready for implantation at the surgical site. Since thedevice is fully preformed and delivered as a unitary implant, the deviceis under direct surgeon control until the disc nucleus prosthesis iscompletely formed. This unitary design reduces the need for the surgeonto determine how to configure the prosthesis to allow for the mostefficacious filling of the evacuated disc nucleus cavity and assuresthat the components' order of insertion and connection are properlyachieved. The ability to predetermine the size of the modular discprosthesis also allows for the evacuated disc nucleus cavity to be morecompletely filled and provides a greater degree of control over theuniformity of the stress response of the implant as compared to otherkinds of minimally invasive implants. In this regard, it will beunderstood that the modular disc prosthesis 100 of the present inventionmay be provided in a variety of different final assembled volumes andshapes to correspond to different sizes and shapes of differentevacuated disc nucleus cavities.

Modular disc prosthesis 100 may be introduced through an access tubethat is inserted partially into the disc nucleus space. Access tube isat least 3 inches long and preferably about 6 inches long. An insertiontool 400 as shown in FIG. 8 may be used to aid in the insertion,positioning, and rail removal of the modular prosthesis. The proximalend of the tool 400 has a handle 402 with an enclosed ratchet or rollermechanism attached to and in line with the inner lumen of an elongatedtube 404 at the distal end of the tool through which a rail 406 may beinserted. The elongated tube 404 may have a slit or other openings alongthe length of the tube 404 to aid in threading the rails 406 into thetube. Insertion tool 400 can then be used to guide modular segments 408into the disc space. The insertion tool 400 may be made out of anycombination of plastics, metals, ceramics, or the like.

It should be noted that although the insertion of modular discprosthesis 100 is described in relation to a preferred five-segmentembodiment or an alternate four-segment embodiment, embodiments havingany other number of segments would be inserted in a similar fashion.

Referring again to FIG. 3, there can be seen a modular disc prosthesis200 prior to insertion into the body. Upon inserting the access tubeinto the disc nucleus space, the first rail 210 is threaded through thelumen of the elongated tube 404 of the insertion tool 400. The insertiontool 400 is then used to push first modular segment 202 through the tubeand into the disc space. Upon complete insertion of first modularsegment 202, modular disc prosthesis 200 is moved centrally and theinsertion tool 400 is repositioned onto the first rail 210 proximal tosecond modular segment 204 which is slid along the first rail 210 intothe evacuated disc nucleus space onto first segment interlocking portion216 until it is flush with first modular segment 202. This stage ofinsertion is depicted in FIG. 4. A stem portion of first rail 210 isthen removed and modular disc prosthesis 200 is moved centrally again.

Second rail 212 is then threaded through insertion tool 400 and thirdmodular segment 206 is slid down second rail 212 and into the discnucleus space onto second segment interlocking portion 218 until it isflush with second modular segment 204. This configuration is shown inFIG. 5. A stem portion of second rail 212 is then removed and modulardisc prosthesis 200 is moved centrally. Third rail 214 is then threadedthrough insertion tool and fourth modular segment 208 is slid alongthird rail 214 into the disc nucleus space and onto third segmentinterlocking portion 220 until it is flush with the other modularsegments 202, 204, 206. Finally, a stem portion of third rail 214 isremoved. This final implanted configuration of modular disc prosthesis200 with all modular segments aligned and locked together is shown inFIG. 6. Modular disc prosthesis 200 is sized and shaped to conform tothe geometry of the evacuated disc nucleus cavity.

In an alternate embodiment, a keystone approach can be used to insertthe modular disc prosthesis such that the last modular segment insertedinto the disc nucleus space is not one of the outside segments. Instead,the outside segments can be the first two segments inserted. Thiscreates a bilateral expansion force as the remaining segments areinserted between the two outside segments. This helps make a tighter fitwithin the evacuated disc nucleus cavity than does the asymmetriclateral force imparted when the segments are implanted sequentially.

The stem portions of rails 110, 112, 114, 115 that extend beyond theperiphery of the body of the modular disc prosthesis 100 can be removedby many different techniques. Insertion tool 400 may be provided with acutting mechanism that can remove the stem portions of the rails. Thecutting mechanism may be a pair of fixed blades located on the distalend of the pushing tool. In this embodiment, the cutting blades wouldact as a cutting wheel in which a turning of the handle connected to theblades causes the blades to circumscribe the rail. Alternatively, thecutting mechanism can be a clamping means that removes the rails throughtwisting or pinching. Stem rails may also be cut off with any othersharp instrument.

In another embodiment, the stem portions of the rails may be providedwith a perforation at the junction with each modular segment such thatthey can be torn, broken, twisted, or more easily cut off. Cutting mayalso be accomplished with a wire loop provided to the part.Additionally, heat, laser, or any other local energy source can be usedto accomplish the separation. One of skill in the art will recognizethat numerous alternative means exist whereby stem rails can be severedfrom the modular disc prosthesis.

Alternatively, the modular disc prosthesis may be implanted using ananterior lateral approach. An anterior lateral approach allows for alarger insertion opening to be used while still being minimallyinvasive. In this approach, the disc is accessed from the patient's sidethrough the psoas muscle, which avoids major nerve and vascular tissues,and may be used in the presence of medical conditions mitigating againstthe posterior approach. This approach is essentially oriented 90° fromthe posterior approach. In this embodiment, it may be acceptable to haveonly two modular segments that comprise the prosthesis with theside-by-side orientation of the segments being generallymedial-to-lateral instead of posterior-to-anterior.

During insertion, slots 128, 130, 132, 133 slide along the stem portionsof rails 110, 112, 114, 115 and onto segment interlocking portions 116,118, 120, 121. Slots 128, 130, 132, 133 and segment interlockingportions 116, 118, 120, 121 may be provided with locking features toprevent separation of modular segments 102, 104, 106, 108, 109. Lockingfeatures, such as a barb or stud or a series of barbs or studs, may beprovided such that once a slot is slid onto a segment interlockingportion, it cannot be slid back off of it. A ratchet and pawl may alsobe used to lock modular segments together. A ratchet release tool mayalso be provided in case separation of modular segments is desired oncethey are locked together.

One example of these locking features is depicted in FIG. 7. Hard innerportion 304 b of each modular segment 304 is provided with a pair ofdepressible projections 334 on segment interlocking portion 318 and acomplementary pair of apertures 336 on slot 328. When slot 328 of afirst modular segment 304 is slid onto segment interlocking portion 318of a second modular segment, projections are depressed. When aperturesof the first modular segment are positioned over projections of thesecond modular segment, the projections pop through apertures 336,locking the modular segments relative to one another. Modular segmentsmay be separated by depressing the projections and sliding the firstmodular segment back off of the second modular segment.

Alternatively, free movement of modular segments 102, 104, 106, 108, 109along rails 110, 112, 114, 115 may be allowed until insertion in thebody. It will be understood that, depending upon the materialconfiguration of the modular prosthesis 100 and the interface fit,segment interlocking portions 116, 118, 120, 121 may swell due tohydration to lock in the final configuration. This feature may be usedalone or in combination with a mechanical locking feature. Alternativemethods of locking modular segments together will be appreciated bythose skilled in the art.

In the preferred embodiment, modular disc prosthesis 100 is molded fromelastomeric biomaterials, preferably polyurethane. Stem rails 110, 112,114, 115 and hard inner portions 102 b, 104 b, 106 b, 108 b, 109 b aremade from a hard durometer polyurethane, such as a polyurethane with aShore D hardness of about 45 or above and compressive modulus in therange of about 70 to 100 MPa. Soft outer portions 102 a, 104 a, 106 a,108 a, 109 a are made from a softer durometer polyurethane, such as apolyurethane with a Shore A hardness ranging from about 40 to 80 and acompressive modulus in the range of about 6 to 20 MPa.

In the preferred embodiment, the two different durometer polyurethanesmay be co-polymerized to create a chemical bond between the two portionsof each modular segment 102, 104, 106, 108, 109. In alternateembodiments, other polymers such as PEEK, polyethylene, silicones,acrylates, nylon, polyacetyls, and other similar biocompatible polymersmay be used for the hard inner portions or the soft outer portions.

In an alternate embodiment, the stem of the tails may be molded from aharder durometer material than soft outer portion and hard inner portionof modular segments. Utilizing this approach allows the rails to beextruded, rather than molded as part of the modular segments. A bondjoint can then be made with the hard inner portion external to theperiphery of the modular segments to form the unitary design. Extrudingthe stem portions of the tails makes the modular disc prosthesis easierand less expensive to manufacture than a completely molded product.

In the preferred embodiment, the modular disc prosthesis is deformablein response to normal physiological forces of 30 to 300 pounds. Becauseof this deformability, the prosthesis produces a physiologicallyappropriate amount of loading on the end plates of the intervertebraldisc. As a result, the end plates will not excessively deform over timeand ultimately conform to the contours of the implant as is the casewith many more rigid disc nucleus replacement implants.

In an alternate embodiment, the outer shell of the modular disc nucleusprosthesis may be modified to provide for elution of medicants. Suchmedicants may include analgesics, antibiotics, antineoplastics orbioosteologics such as bone growth agents. While motion preservation isgenerally a principle goal in nucleus replacement, in certainindications it may be desirable to promote some bony fusion. Suchindications may include nucleus replacements in the cervical spine.

The solid polymer outer shell of the modular disc nucleus prosthesis mayprovide for better and more controllable elution rates than somehydrogel materials. In an alternate embodiment, the modular disc nucleusprosthesis may include different elution rates for each polymermaterial. This would allow for varying elution rates for differentmedicants.

Various modifications to the disclosed apparatuses and methods may beapparent to one of skill in the art upon reading this disclosure. Theabove is not contemplated to limit the scope of the present invention,which is limited only by the claims below.

1. A minimally invasive method of implanting a modular disc prosthesis into an evacuated disc nucleus space, the method comprising: inserting a first modular segment into the disc nucleus space through an opening, the first modular segment having a width with a first rail that extends out of the disc nucleus space when the first modular segment is located within the disc nucleus space; sliding a second modular segment along the first rail into the disc nucleus space through the opening until the second modular segment interlocks with the first modular segment, the second modular segment having a width with a second rail that extends out of the disc nucleus space when the second modular segment is located within the disc nucleus space; removing a portion of the first rail that extends from the interlocked first and second modular segments; sliding a third modular segment along the second rail into the disc nucleus space through the opening until the third modular segment interlocks with the second modular segment, the third modular having a width; and removing a portion of the second rail that extends from the interlocked second and third modular segments to form an implanted modular disc prosthesis having a generally continuous periphery that corresponds generally to the evacuated nucleus disc space and having a total width at least equal to the widths of the first, second and third modular segment and greater than a width of the opening that is substantially only the width of a largest of the widths of any one of the modular segments.
 2. The minimally invasive method of implanting a modular disc prosthesis of claim 1, the third modular segment having a third rail that extends out of the disc nucleus space when the third modular segment is located within the disc nucleus space, the method further comprising: sliding a fourth modular segment along the third rail into the disc nucleus space through the opening until the fourth modular segment interlocks with the third modular segment, the fourth modular segment having a width; and removing the portion of the third rail that extends from the interlocked third and fourth modular segments.
 3. The minimally invasive method of implanting a modular disc prosthesis of claim 2, the fourth modular segment having a fourth rail that extends out of the disc nucleus space when the fourth modular segment is located within the disc nucleus space, the method further comprising: sliding a fifth modular segment along the fourth rail into the disc nucleus space through the opening until the fifth modular segment interlocks with the fourth modular segment, the fifth modular segment having a width; and removing the portion of the fourth rail that extends from the interlocked fourth and fifth modular segments.
 4. The minimally invasive method of implanting a modular disc prosthesis of claim 1, wherein the steps of sliding the second and third modular segments are performed using an insertion tool and the steps of removing the first and second rails is accomplished by using a cutting mechanism provided at the distal end of the insertion tool. 